Over the past few decades, demand for semiconductor devices has grown rapidly. Semiconductor manufacturers are often pressured into improvements in end-product quality, speed and performance, as well as improvements in manufacturing process quality, speed and performance. Machine vision has proven to be a very essential part of improving the productivity and quality of semiconductor production. There is a consistent drive for faster and more accurate machine vision systems for ever-higher semiconductor yields. Correspondingly, the technical field of three-dimensional measurement and inspection for semiconductor devices, such as semiconductor wafers or substrates on final packaged products, has seen rapid growth.
A straightforward way to improve machine vision accuracy is to increase the aperture size of the lens for higher optical resolution. However, increasing aperture size will lead to decreasing Depth of Field (DOF). This approach conflicts with the trend of three-dimensional semiconductor packaging and requires very accurate placement of devices. As disclosed in U.S. Pat. No. 6,320,979 entitled “Depth of Field Enhancement”, this dilemma can be solved by motorized focusing by way of a lens system that is movably attached to a camera housing, but at the cost of more complex mechanical design, vibration sensitivity and slower response time.
On-the-fly grabbing technology is becoming popular to improve machine vision speed, while the allowed exposure time is very limited to avoid blurring the grabbed image during fast scanning. Due to its limited lighting power, LED lighting may not be applicable in some applications. An on-the-fly inspection system using a Xenon strobe lamp is disclosed in US Patent Publication No. 2000/6049384 entitled, “Method and Apparatus for Three Dimensional Imaging Using Multi-phased Structured Light”. However, a Xenon lamp is bulky and its lifetime is relatively short. On the other hand, LEDs present many advantages over other light sources including higher energy efficacy, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. Thus, LED lighting is preferable in a machine vision system.
Moreover, many high-density semiconductor packaging inspection applications also require three-dimensional measurement capability. Interferometry is quite commonly applied for three-dimensional measurement. An interference fringe pattern results from an optical path difference between a measured object and an internal reference surface. While interferometry has high precision in the order of nanometers, it requires scanning to increase its measuring range. Hence, many measurements may be required to calculate the height of a single surface at a relatively slow speed.
In laser triangulation, a laser projects light onto an object surface and a position sensor is inclined with respect to the incident light. There is a drift in a position of the position sensor when the height varies. Height information can be measured from the drift position on the position sensor. The problem is that errors are introduced by a spot size of the laser light and any non-uniformity of the measured object.
Confocal optical devices make use of the principle that the output signal is at a peak (in intensity or contrast) at a focal plane of the confocal optical device. FIG. 1 is a conventional design of a confocal optical device 100. The confocal optical device 100 is positioned vertically over an object 102 in order to inspect the object 102. A light source 104 projects light rays 108 through a light source pinhole aperture 106. The light rays 108 are passed through a beam splitter 110 and objective lens 112 onto the object 102 which is aligned along a focal plane 120 of the confocal optical device 100. In-focus light rays 114 are transmitted from the object 102 through the objective lens 112 and beam splitter 110 and pass through a detector pinhole aperture 116. Only in-focus light rays 114 which pass through the detector pinhole aperture 116 are collected by an image sensor 118, whereas out-of-focus light rays are mostly blocked. Thus, the confocal optical device 100 is efficient at rejecting out of focus light using the detector pinhole aperture 116. A high contrast image comes from a small section of the inspected object (at the focal plane 120 of the confocal optical device 100) since it has a small depth of field.
For measuring variations in height, the confocal optical device needs to scan the whole range of possible heights by either moving the imaging system 100, or changing the height of the object 102. Therefore, the measuring speed is very slow.
Another example of a prior art approach to measuring a height of a workpiece is disclosed in US Patent Publication No. 2008/0204748 A1 entitled, “Measuring Device for Workpiece Held on Chuck Table”. A measuring device is used for measuring a height of a workpiece which is held on a chuck table. A white light source emits white light while an acousto-optical device deflects the white light to separate the white light to produce a flux of diffracted light of varying wavelengths. A voltage is controlled to obtain different colors of light, and a pinhole mask passes part of the light having certain wavelengths through it. A chromatic aberration lens is configured to focus flux of different colors of light passing through the pinhole mask onto the workpiece. A photodetector is configured to detect the light reflected from the workpiece through the chromatic aberration lens. Through a map of voltage against a measured height (obtained from calibration), a height of the workpiece is determined from a value of voltage which gives rise to a peak intensity detected by the photodetector. However, the method is sensitive to color change on a surface of the measured workpiece.
Yet another example of a prior art approach is disclosed in US Patent Publication No. 2009/0277889 A1 entitled, “Laser Beam Machining Apparatus with Detection Laser Beam Oscillator”. A height position of an upper surface of a workpiece held on a chuck table is detected by projecting a beam of laser light with a narrow focal range onto the workpiece. The laser light reflected from the workpiece is detected through focusing lens with a narrow depth of field. Through the ratio of focusing scores measured from two photodetectors, a height of the measured device can be obtained. A shortcoming of this approach is that it is sensitive to a contrast or texture of the surface of the workpiece.
It would be advantageous to conduct three-dimensional measurement more effectively while avoiding at least some of the aforesaid disadvantages of the prior art.